The replicative form (RF) DNA of the Parvovirus, H-1, is being used as a molecular probe of mammalizn cell DNA replication. Evidence indicates replication of RF DNA is solely host mediated and its initiation is dependent on cell proteins synthesized in late S phase. Once begun, replication proceeds at an exponential rate and is not dependent on concomitant protein synthesis for either initiation or continuation of the process. In vitro methods that are capable of initiating and continuing the replication of H-1 RF DNA are being sought. An attempt is being made to reduce the RF DNA to the samllest unit (replicator) capable of replication. Success in these efforts will be followed by attempts to replicate DNA fragments derived from oncogenic viruses through linkage to the parvovirus replicator.